1. Field of the Invention
The present invention relates to an apparatus and method for the in-furnace reduction of nitrogen oxide emissions in flue gas using natural gas and/or other fuels as the reducing agent.
2. Description of the Prior Art
In the combustion of fuels with fixed nitrogen such as coal, oxygen from the air may combine with the nitrogen to produce nitrogen oxides. At sufficiently high temperatures, oxygen reacts with atmospheric nitrogen to form nitrogen oxides. Production of nitrogen oxide is regarded as undesirable. Nitrogen oxides are toxic, they contribute to acid rain and can make rain, dew and mist corrosive. There are numerous government regulations which limit the amount of nitrogen oxide which may be emitted from a combustion furnace. Consequently, there is a need for apparatus and processes which reduce the nitrogen oxide emissions in furnace flue gas.
Numerous attempts have been made to develop apparatus and processes which reduce the nitrogen oxide emissions in a furnace flue gas. One such approach is a process known as in-furnace nitrogen oxide reduction, reburning, or fuel staging. In reburning, coal, oil, or gas is injected above the normal flame zone to form a fuel-rich zone. In this zone, part of the nitrogen oxides are reduced to ammonia and cyanide-like fragments and N.sub.2. Subsequently, air is injected to complete combustion. The reduced ammonia and cyanide-like fragments are then oxidized to form N.sub.2 and nitrogen oxide.
Several problems occur when this process is used. First, coal may be an inefficient reburn fuel because of its high fixed-nitrogen composition. Within any furnace there are wide temperature zones in which fuel nitrogen will convert to nitrogen oxide. Thus, the fixed nitrogen reduced from the coal has a chance of. ending up as nitrogen oxide.
Furthermore, the fuel must be injected with a sufficient volume of gas. If air or flue gas containing oxygen is used as this carrier gas, there must be enough fuel to consume the oxygen in the carrier, and to supply an excess of fuel so reducing conditions exist. This increases the amount of fuel which must be used as reburn fuel. Furthermore, the necessity of using carrier air requires extensive duct work in the upper part of the furnace.
Additionally, the reburn fuel must be injected well above the primary combustion zone of the furnace so that it will not interfere with the reactions taking place therein. However, this fuel must be made to burn out completely without leaving a large amount of unburned carbon. To do this, the fuel must be injected in a very hot region of the furnace some distance from the furnace exit. The exit temperature of the furnace must be limited in order to preserve the heat exchangers, surface. Therefore, a tall furnace is required to complete this second stage process.
Moreover, the fuel must be injected in such quantities as to make the upper furnace zone fuel rich. This fuel is supplied in excess to the amount of air in the furnace and ultimately requires more air in order to be completely combusted. Thus, air must be injected above the reburn fuel injection. This requires even more duct work and furnace volume.
Finally, most coal furnaces which are now in operation are not designed to accommodate the prior art methods. Major modifications such as the provision of extensive ductwork and the addition of a second stage to the process are required to utilize the prior art method. Such retrofitting is expensive. Consequently, there is a need for a combustion apparatus and process which will reduce nitrogen oxide emissions in flue gas and which can be readily used in existing furnaces.
In our U.S. Pat. No. 4,779,545, a reburn process is disclosed wherein natural gas is introduced into the upper furnace through pulse combustors. The patent teaches that the natural gas must be injected in pulses to achieve NO.sub.x reduction. This process does not require any carrier air or flue gas for NO.sub.x reduction. However, it does require the expense of obtaining and operating pulse combustors and some air may be required. Therefore, there is a need for an improved process for in-furnace reduction of nitrogen oxides which can be implemented at low cost.
In our U.S. patent application Ser. No. 608,718, we disclose an apparatus and process for reducing nitrogen oxide which employs pipes, orifices and nozzles to introduce reburn fuel into the upper part of the furnace with sufficient turbulence to cause rapid mixing. In our U.S. patent application Ser. No. 623,782, an apparatus and a process is disclosed wherein pipes, orifices, nozzles, diffusers, ceramic socks and porous ceramic bodies are employed to allow the reburn fuel to diffuse slowly into the flue gas. Although these techniques work they cannot be precisely controlled. We have now discovered that fuel injected in the form of vortices, such as vortex rings, can be directed and controlled so the maximum reduction of nitrogen oxide can be obtained.